Spectroscopic studies of the activation of chemical reactivity of dioxygen by iron porphyrins in both model systems and in intact, functioning proteins are proposed. These studies are designed to study the mechanism of action of a number of heme utilizing proteins including peroxidases, catalases, cytochrome P-450, cytochrome-c oxidase and heme oxidase. In model studies, intermediates, particularly those involving the (FeO)2+ and (FeO)3+ groups, which have been previously proposed as important to the of action of these proteins will be prepared at low temperature and their their electronic structure and chemical reactivity will be examined. This involve spectgroscopic examination particularly by 1H-NMR spectroscopy, but but also be electronic, infrared and electron spin resonance spectroscopy. study of proteins involves 1H-NMR studies of paramagnetic proteins in their their native and reactive forms. Model studies will focus on the chemical transformation of the five-coordinate PFeO2 (P=porphyrin dianion), attempts to stabilize PFeO and examine its reactivity as an oxidant, studies of the interconversion of the (FeO)2+ and (FeO)3+ groups, studies of the axial coordination and oxidation states of iron oxyphlorins, studies of porphyrin N-oxides and their ability to act as hydroxylating agents, and studies of the reactions between peroxides and iron porphyrins. Studies on proteins will focus on further definition of the electronic structure of horseradish peroxidase compound I, further comparision of horseradish peroxidase compound II with models containing the (FeO)2+ unit, and identification of the free radical center in cytochrome c peroxidase as well as studies of the influence of hydrogen bonding near the active site. 1H-NMR studies on cytochrome P-450 will characterize the heme environment with regard to axial ligation, heme orientation, side chain mobility and ionizable protons. Direct detection of reactive intermediates and comparison with models will be attempted.